1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of the Related Art
An optical encoder comprises a main scale and a detection head for optically reading the main scale, which are assembled together. On assembling such the optical encoder, attitude adjustments are important to achieve a high performance. Specific attitude adjustments are required for: a gap between the main scale and the detection head; a rotation about each axis between the detection head and the main scale; and an offset in a direction perpendicular to a measurement axis between the detection head and the main scale.
In particular, a small optical encoder has optical gratings formed with a fine pitch and thus disadvantageously requires assembling and attaching with a high precision to achieve a high performance. Mechanical adjustments for attitude variations have been performed in the art but have limitations in mechanical attitude adjustments.
The present invention has been made in consideration of the above situation and accordingly has an object to provide an optical encoder capable of achieving a high performance without the need for attitude adjustments and assembling with a high precision.
The present invention is provided with an optical encoder, which comprises a main scale having first optical gratings formed thereon with a certain angular pitch along a measurement axis. The measurement axis describes a circular arc. The encoder also comprises a detection head having second optical gratings formed thereon. The detection head is arranged opposing to the main scale and is relatively movable along the measurement axis to detect relative displacement between said main scale and said detection head. The second optical gratings on the detection head are arranged with a certain distance pitch on a straight line.
The present invention is also provided with an optical encoder, which comprises a main scale having first optical gratings formed thereon with a certain distance pitch along a measurement axis. The measurement axis describes a straight line. The encoder also comprises a detection head having a second optical gratings formed thereon. The detection head is arranged opposing to the main scale and is relatively movable along the measurement axis to detect relative displacement between said main scale and said detection head, The second optical gratings on the detection head are arranged with a certain angular pitch on a circular arc.
In the case where the optical encoder of the present invention is a rotary type with a measurement axis that describes a circular arc, the optical gratings on the detection head has a grating arrangement on a straight line. In the case where the optical encoder is a linear type with a measurement axis that describes a straight line, the optical gratings on the detection head has a grating arrangement on a circular arc. According to such the arrangements, the optical gratings on the main scale overlap the optical gratings on the detection head, varying the overlapping state little by little from adjacent one, within a certain length along the measurement axis of the detection head. As a result, even if a misalignment occurs in an attitude of one to another between opposite optical gratings, the misalignment of the attitude less varies the overlapped state of optical gratings, hardly suffering affection from attitude fluctuations.
In the present invention, the two opposite optical are designed respectively in a circular arc arrangement and a straight line arrangement. Therefore, as they are apart from the center of the detection head along the measurement axis, an oblique angle between the two optical gratings increase gradually and an overlapping area between the two optical gratings decreases, resulting in an increase in the DC offset component of the output signal. To improve this situation, it is efficient to restrict an effective photodetective surface of the optical gratings on the detection head within a certain length, along the measurement axis, in which the two optical gratings have almost identical pitches. An aperture may be useful for defining the effective photodetective surface. The aperture is shaped, for example, to have sizes larger at the center of the certain length of the second optical gratings and smaller at peripheries along the measurement axis. Such the aperture can suppress large DC light components produced at both ends of the optical gratings along the measurement axis on the detection head and ensures a sufficiently large ratio of peak-to-peak value VPP to DC offset VDC of the output signal.
In the present invention, the detection head may comprise either: (a) an index scale having the second gratings formed thereon and photodetective devices for receiving lights transmitted through the second gratings; or (b) a photodetective device array that serves as the second gratings.